Gelatin is used as a binder for various photographic materials. For example, silver halide photographic emulsion layers, protective layers for emulsion layers, filter layers, interlayers, antihalation layers, backing layers, film base subbing layers, baryta layers, etc., of photographic materials use gelatin as the main constitutional component.
Photographic materials containing gelatin are processed using various aqueous solutions having various pH and temperature values. If the layers of photographic materials contain gelatin which is not treated with a hardening agent, the properties of these layers depend mainly upon the properties of gelatin, and are poor in water resistance, excessively swell in an aqueous solution to greatly reduce the mechanical strength. In an extreme case, the gelatin layer is dissolved off in an aqueous solution, e.g., in a high temperature solution (higher than 30.degree. C.) or in a high-alkaline aqueous solution. Such properties are fatal defects as properties of layers constituting photographic materials.
For increasing the water resistance, heat resistance and mechanical strength of a gelatin layer by hardening gelatin, it has hitherto been known that many compounds are effective.
These compounds are known as "hardening agents", and are usually used for the production of photographic materials. They include, for example, aldehyde series compounds such as formaldehyde, glutaraldehyde, etc., compounds having a reactive halogen described, e.g., in U.S. Pat. No. 3,288,775, compounds having a reactive ethylenically unsaturated bond described in, e.g., U.S. Pat. No. 3,642,486, Japanese Patent Publication No. 13,563/'74, aziridine series compounds described, e.g., in U.S. Pat. No. 3,017,280, etc., epoxy compounds described in U.S. Pat. No. 3,091,537, halogencarboxylaldehydes such as mucochloric acid, etc., dioxanes such as dihydroxydioxane, dichlorodioxane, etc., and inorganic hardening agents such as chromium alum, zirconium sulfate, etc.
However, when these known gelatin hardening agents are used for photographic materials, they have defects in that the hardening action is insufficient, the degree of hardening changes for a long period of time due to "post-hardening" (caused by slow continued hardening of the gelatin, bad actions (such as, in particular, the increase of fog, the reduction of sensitivity, etc.) are given to the properties of photographic materials, effects of the hardening agents are reduced by other photographic additives which coexist with the hardening agents, or they reduce the effects of other photographic additives (e.g., couplers for color photographic materials), etc.
Hardening agents showing a relatively high hardening reaction rate and less post hardening include compounds having a dihydroquinoline skeleton as described in Japanese Patent Application (OPI) No. 38,540/'75, corresponding to U.S. Pat. No. 4,013,468 (the term "OPI" used herein indicates an unexamined published patent application open to public inspection); compounds having a phosphorus-halogen bond as described in Japanese Patent Application (OPI) No. 113,929/'83; compounds having an N-sulfonyloxyimido group as described in Japanese Patent Application (OPI) No. 93,470/'77, corresponding to U.S. Pat. No. 4,111,926; and compounds having two or more N-acyloxyimino group in one molecule as described in Japanese Patent Publication No. 22,089/'78, corresponding to U.S. Pat. No. 4,052,373. These hardening agents have excellent properties such that the hardening action proceeds quickly and the occurrence of the so-called post hardening is greatly lessened, but they are poor in water-solubility, whereby they are liable to cause non-uniform hardening in silver halide photographic emulsion layers. Furthermore, since these hardening agents require specific organic solvents in the case of adding the hardening agents to silver halide photographic emulsions or silver halide photographic emulsion layers, the trouble of uneven coating is liable to cause based on the organic solvent as well as the use of an organic solvent requires care for the prevention of explosion.
Hardening agents showing a high hardening reaction for gelatin and having an excellent water solubility include N-carbamoylpyridinium salts as described in Japanese Patent Application (OPI) Nos. 51,945/'74 and 59,625/'76 corresponding respectively to U.K. Pat. No. 1,383,630 and to U.S. Pat. No. 4,063,952; and 2-sulfonyloxypyridinium salts as described in Japanese Patent Application (OPI) No. 110,762/'81.
These hardening agents having features such that a water solubility is high, the hardening action for gelatin is fast, and the occurrence of post-hardening is less. However, these hardening agents show a high hardening reaction rate for gelatin and a side reaction occurs quickly by the decomposition with water. These hardening agents have the fault that the effective use efficiency of the hardening agent is very low in a general production method of photographic materials using an aqueous solution of gelatin; therefore, a large amount of the hardening agent must be used for obtaining gelatin layers having a desired degree of hardening.
It is generally considered that when the hardening agents such as described in Japanese Patent Application (OPI) Nos. 51,945/'74; 59,625/'76 and 110,762/'81 are nucleophilically attacked by a carboxy group or an amino group of gelatin, the hardening agents react with the group to harden the gelatin. In the case of producing gelatin layers, an aqueous solution of gelatin is generally used. In this case, since water has a nucleophilic property to some extent, water reacts with the foregoing hardening agent to decompose the hardening agent, whereby the effect of the hardening agent is inevitably lost. This tendency is particularly severe for a hardening agent showing a fast hardening action.
Accordingly, in order to efficiently and quickly harden gelatin, the development of a hardening agent showing a higher reactivity with a carboxy group or an amino group of gelatin than the reactivity with water in gelatin, that is, having a good selectivity in the reaction with gelatin, is important. Hardening agents such as N-carbamoylpyridinium salts and 2-sulfonyloxypyridinium salts have a fault in that the effective use efficiency of the hardening agent is very low owing to the low selectivity in the reaction with gelatin. Therefore, the development of a hardening agent showing a high effective use efficiency, showing a fast harden reaction rate with gelatin, and having a high water solubility has been ardently desired.